Avoiding Fertilizer Burn

The claim most often made for organic fertilizers is that they "do
not burn". Fertilizer burn means injury (dehydration) of the roots
or crown of the plant (plant burn), and a browning of part or all of the
foliage, sometimes resulting in the death of the entire plant.

The tissues are not "burned" as in a bonfire; the burn is a
drying-out caused by the withdrawal of water from tissues by the 'hygroscopic'
action of "thirsty" chemical materials.

Since some manufacturers
make a big issue of this "non-burning" quality, many home gardeners
have come to regard it as an important factor in selecting a fertilizer
product for use. After all, who wants lawn or plant burn.

The identical
property which makes food elements available as plant nutrients - high
solubility - will also make them likely to "burn" or injure
plant tissues. The burn may be immediate or it may be delayed, but if
the soluble ingredient which causes this condition is present in excess
amounts, burn is almost inevitable.

A 'Sod' Story

On lawns, for example, a burn from an overdose of a chemical fertilizer
such as ammonium sulfate becomes visible almost at once, while a too-heavy
application of sewerage sludge may not show any ill effects for a month
or two after application to the turf.

Unfortunately, the burn from organic
sources comes so much later after application that it is seldom traced
to its cause. Nevertheless, because it happens in summer - a time when
grass is peculiarly susceptible to severe injury - a delayed organic nitrogen
burn of this kind usually causes more permanent injury than does a burn
inflicted early in the season by a chemical fertilizer.

To understand how all this happens let us look at two lawn areas, one
fertilized heavily with a highly soluble chemical like ammonium nitrate
and the other with sewerage sludge (a typical organic fertilizer).

Lawn 1

In the first instance, the owner applies 10 pounds of ammonium nitrate in
spring soon after grass begins to grow. He fails to water it in, and so
the chemical fertilizer begins to suck moisture out of the grass plants
to satisfy its "thirst."

In a matter of a few days, the entire
lawn is dappled brown and green - severe nitrogen burn.

After the owner realizes his mistake, he waters heavily to wash out the excess salts;
the grass gradually recovers and turns dark green. By June it is healthy
and ready for another feeding. This time the owner waters it in and no
burn results.

Lawn 2

The same week in early spring the second lawn, perhaps next door to the
first and on the same type of soil, is fed with sewerage sludge because
the owner is convinced this organic fertilizer will not burn.

He applies 50 pounds per 1,000 square feet of lawn. April and May remain cool, however,
and because soil bacteria are inactive the organic material is not broken
down and this first application produces no results.

The owner, certain he needs more fertilizer, dumps on another 25 pounds of sludge per 1,000
square feet. The two applications contribute less than four pounds of
actual nitrogen per 1,000 square feet, not an excessive amount by any
means. If organic nitrogen were really non-burning, he would have no trouble.

Unfortunately for him, the theory doesn't hold water. About the first
week in June, hot weather comes on suddenly. Soil bacteria that have been
all but dormant begin to multiply at an unbelievable rate.

In a matter of two weeks, nitrogen is being released from the sludge-rich soil to
the plants so rapidly that grass blades wilt and collapse. If a nitrogen
test chemical is applied to the grass at this time, tiny beads of red
will show up on every blade.

This indicates that free nitrogen is being
released - actually exuded from the blades - by a process known as guttation.

This lawn is suffering from a far more serious nitrogen burn than lawn
number one received in Spring. About as close as the second lawn owner
will come to knowing the real cause, however, will be his comment, "I
guess my grass just couldn't stand summer heat."

A Familiar Problem

Golf course greenskeepers are familiar with this problem. Because they
must always maintain grass in top condition, they often have to push turf
feeding close to capacity and risk bringing on guttation.

The moment they see traces of grass blade wilting or "flagging"
they will apply test chemicals to see if nitrogen is coming out of the
foliage. If it is, a crew of men is quickly assigned to wash (leach) the
excess plant food out of the soil with liberal applications of water.

Avoid Excess

A paramount fact every gardener should fix in mind is that any excess
of nitrogen beyond the needs of the plant will cause plant burn. This
burn may become visible very soon after the fertilizer is applied or,
in the case of manures, the effect may be delayed many weeks.

Even certain organic products can cause rapid burn if used in excess.
Dried blood, one of the most valuable sources of nutrients, is so readily
soluble that it may work like a chemical fertilizer salt. The same is
true, to a somewhat lesser degree, of fish emulsions and urine.